> why does any app need to know which other apps I have installed, and how long I use them

What are you referring to? As far as I know, one app can't query the list and usage time of other installed apps on iOS. This stackoverflow question[1] seems to confirm this. But please enlighten me if I'm misunderstanding what you're referring to.

[1] https://stackoverflow.com/questions/51634436/how-to-get-a-li...